Method of heat treating steel

ABSTRACT

A method of heat treating a steel workpiece may include two heat treatment stages. In the first stage, the steel workpiece may be heated to a first temperature greater than or equal to its A 1  temperature, but less than its A 3  temperature to transform the microstructure of the steel workpiece into a multiphase microstructure including grains of ferrite and grains of austenite having an average grain diameter. In the second stage, the steel workpiece may be heated to a second temperature greater than the first temperature to increase the average grain diameter of the grains of austenite. Thereafter, the steel workpiece may be cooled to ambient temperature at a rate sufficient to retain a major portion of the grains of austenite obtained during the first and second heat treatment stages. The as-heat treated steel workpiece may comprise a retained austenite phase dispersed within a ferrite matrix phase at ambient temperature.

TECHNICAL FIELD

The present disclosure relates to methods of heat treating steel and, more specifically, to methods of heat treating steel workpieces to improve the deformation behavior thereof.

INTRODUCTION

Steel is commonly used in the manufacture of automotive vehicle body panels and support structures, including frames, rails, crossmembers, pillars, roofs, quarter panels, hoods, and deck lids, to name a few. Various forming processes, including drawing, stamping, and rolling, may be used to plastically deform a steel workpiece for the purpose of strengthening the workpiece and/or forming the into a desired shape. Depending on its composition and microstructure, in some loading situations, the steel workpiece may experience heterogeneous plastic deformation, with the deformed regions of the workpiece undesirably manifesting themselves as depressions known as Lüders bands or stretcher strain marks on the surface of the steel workpiece. Various methods of promoting the homogenous deformation of steel workpieces have been developed for the purpose of eliminating the formation of Lüders bands or stretcher strain marks during steel forming operations. However, such methods oftentimes result in a loss of ductility and/or mechanical strength and may only improve the uniform deformation behavior of the steel workpiece for a limited duration.

SUMMARY

A method of heat treating a steel workpiece having a polycrystalline microstructure. In step (a), the steel workpiece may be heated to a first temperature greater than or equal to its lower austenite transformation temperature A₁, but less than its upper austenite transformation temperature A₃ to transform the microstructure of the steel workpiece into a multiphase microstructure including grains of ferrite and grains of austenite having an average grain diameter. In step (b), the steel workpiece may be heated to a second temperature greater than the first temperature to increase the average grain diameter of the grains of austenite in the steel workpiece. In step (c), the steel workpiece may be cooled to ambient temperature at a rate sufficient to retain a major portion of the grains of austenite obtained during steps (a) and (b). The resulting microstructure of the heat treated steel workpiece may comprise a retained austenite phase dispersed within a ferrite matrix phase at ambient temperature.

The steel workpiece may comprise, by weight, 5-12% manganese (Mn) and 0.1-0.3% carbon (C).

The steel workpiece may be heated in step (a) to a first temperature less than or equal to 50° C. below the A₃ temperature of the steel workpiece.

The steel workpiece may be heated in step (b) to a second temperature greater than or equal to 100° C. below the A₃ temperature of the steel workpiece and less than or equal to 20° C. above the A₃ temperature of the steel workpiece.

The steel workpiece may be heated in step (a) for a duration in the range of one second to one-hundred hours. In addition, the steel workpiece may be heated in step (b) for a duration in the range of one second to 1000 seconds.

The steel workpiece may be cooled to a third temperature less than the first temperature between steps (a) and (b).

After step (c), 10-100% of the grains of austenite in the steel workpiece may have a diameter greater than an average grain diameter of the ferrite matrix phase.

After step (c), the retained austenite phase may comprise at least 30 vol % of the microstructure of the steel workpiece.

After step (c), the ferrite matrix phase may comprise at least 40 vol % of the microstructure of the steel workpiece.

After step (c), the steel workpiece may comprise ≤10 vol % martensite, bainite, pearlite, and/or cementite.

After step (c), the steel workpiece may be formed into a shaped part without exhibiting Lüders strain or yield point elongation during deformation thereof.

The steel workpiece may be in the form of a hot-rolled and cold-rolled steel sheet.

A method of manufacturing a steel part. A hot-rolled and cold-rolled steel sheet may be provided. The steel sheet may have a polycrystalline microstructure and may comprise, by weight, 0.1-0.3% carbon (C) and 5-12% manganese (Mn). The steel sheet may be heated to a first temperature greater than or equal to its lower austenite transformation temperature A₁, but less than its upper austenite transformation temperature A₃ to transform the microstructure of the steel sheet into a multiphase microstructure including grains of ferrite and grains of austenite having an average grain diameter. The steel sheet also may be heated to a second temperature greater than the first temperature to increase the average grain diameter of the grains of austenite in the steel sheet. Then, the steel sheet may be cooled to ambient temperature at a rate sufficient to retain a major portion of the grains of austenite within the microstructure of the steel sheet so that the steel sheet comprises a retained austenite phase dispersed within a ferrite matrix phase at ambient temperature. Thereafter, the steel sheet may be formed into a shaped steel part.

The hot-rolled and cold-rolled steel sheet may have a thickness in the range of 0.5 mm to 6 mm.

The steel sheet may be formed into a shaped steel part without exhibiting Lüders strain or yield point elongation during deformation thereof.

After forming, an exterior surface of the shaped steel part may be free of Lüders bands or stretcher strain marks.

The shaped steel part may comprise an automotive vehicle body panel or support structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a multistage process for heat treating a low carbon, medium manganese steel workpiece, in accordance with one aspect of the present disclosure;

FIG. 2 is a schematic illustration of a process for manufacturing a shaped steel part from a hot rolled, cold rolled, and heat treated steel workpiece, in accordance with one aspect of the present disclosure;

FIG. 3 is a graph of Temperature vs. Time illustrating a process for manufacturing a low carbon, medium manganese steel workpiece that includes a multistage heat treatment process, in accordance with one aspect of the present disclosure;

FIG. 4 is a graph of Temperature vs. Time illustrating a process for manufacturing a low carbon, medium manganese steel workpiece that includes a multistage heat treatment process, in accordance with another aspect of the present disclosure;

FIG. 5 is an electron backscattered diffraction (EBSD) image illustrating the microstructure of a low carbon, medium manganese steel sheet that has been subjected to a single stage heat treatment process;

FIG. 6 is an electron backscattered diffraction (EBSD) image illustrating the microstructure of a low carbon, medium manganese steel sheet that has been subjected to a multistage heat treatment process; and

FIG. 7 is a graph of Engineering Stress (MPa) vs. Engineering Strain (%) illustrating the deformation behavior of the steel samples cut in a rolling direction from the steel sheets shown in FIGS. 5 and 6.

DETAILED DESCRIPTION

The presently disclosed heat treatment process can be used to produce a low carbon, medium manganese steel workpiece having a multiphase microstructure that provides the workpiece with an excellent combination of high mechanical strength and ductility and with the ability to be formed into a desired shape by a variety of hot and cold forming processes without the formation of Lüders bands or stretcher strain marks on an exterior surface thereof.

FIG. 1 schematically depicts a multistage process 10 for heat treating a steel workpiece 12, in accordance with one aspect of the present disclosure. The steel workpiece 12 depicted in FIG. 1 comprises a continuous sheet or strip of hot rolled and cold rolled steel that is uncoiled from an uncoiler 14, directed in a continuous fashion from one stage of the process 10 to another by a plurality of rollers 16, and then recoiled by a recoiler 18. However, in other embodiments, the steel workpiece 12 may comprise an individual steel blank that may be heat treated in combination with one or more other steel blanks in a continuous or batch process. The steel workpiece 12 may be formed from a slab of steel (not shown) that has been hot rolled at a temperature in the range of 800−900° C. to a thickness in the range of 6 mm to 10 mm, cooled to ambient temperature (20° C.), and then cold rolled to a thickness in the range of 0.5 mm to 6 mm. Corrosion resistance may be imparted to the steel workpiece 12 by applying a layer of zinc to at least one major surface thereof. The layer(s) of zinc may be applied by an electro-galvanizing, galvannealing, or hot-dip galvanizing process, wherein the steel workpiece 12 is immersed in a bath of molten zinc.

The steel workpiece 12 may comprise a steel alloy including one or more alloying elements and the balance iron (Fe). For example, the steel workpiece 12 may comprise an alloy of iron (Fe), carbon (C), and manganese (Mn), wherein carbon and manganese are the largest constituents of the alloy other than iron. The presence of carbon and manganese within the alloy may help increase the thermal stability of austenite grains within the workpiece 12 during the presently disclosed heat treatment process so that an austenite phase may be retained within the microstructure of the steel workpiece 12 at ambient temperature. In one form, the steel workpiece 12 may comprise a steel or iron alloy including, by weight, 0.1-0.3% carbon (C) and 5-12% manganese (Mn). Steel alloys comprising, by weight, greater than 5% manganese, but less than 12% manganese may be referred to as “medium Mn” steels. Steel alloys comprising, by weight, ≤0.3% carbon may be referred to as “low carbon” steels. In one specific example, the steel workpiece 12 may comprise, by weight, 0.14% carbon and 7.0% manganese. The steel workpiece 12 may comprise one or more additional alloying elements. In one form, the steel workpiece 12 may comprise, by weight, ≤2% chromium (Cr), molybdenum (Mo), vanadium (V), niobium (Nb), nickel (Ni), silicon (Si), phosphorus (P), aluminum (Al), nitrogen (N), boron (B), and combinations thereof. For example, the steel workpiece 12 may comprise, by weight, 0.1-2% silicon and/or 0.01-0.5% chromium.

The process 10 depicted in FIG. 1 includes a first heat treatment stage 20, a second heat treatment stage 22, and a subsequent cooling stage 24. The process 10 also may include a cleaning stage 26 prior to the first heat treatment stage 20 to remove grease and grit from the workpiece 12. In addition, the process 10 may include an entry looper or accumulator 28 and an exit looper or accumulator 30 to ensure the continuous and constant velocity operation of the intermediate heat treatment and cooling stages 20, 22, 24, even when the uncoiler 14 and/or the recoiler 18 are stopped or operating at different velocities. The heat treatment and cooling stages 20, 22, 24 may be carried out in a single furnace 32 having three (3) interconnected chambers. Or the heat treatment and cooling stages 20, 22, 24 may be carried out in multiple discrete furnaces (not shown) to allow for the inclusion of one or more intermediate stages between the first and second heat treatment stages 20, 22 and/or between the second heat treatment stage 22 and the cooling stage 24.

Prior to the first heat treatment stage 18, the steel workpiece 12 may exhibit a polycrystalline microstructure including one or more of the following phases: martensite, ferrite, bainite, retained austenite, pearlite, and/or cementite. In one form, the polycrystalline microstructure of the steel workpiece 12 may comprise ≥50 vol. % martensite. For example, the steel workpiece 12 may exhibit a martensitic microstructure prior to the first heat treatment stage 18 and may consist essentially of martensite. The heat treatment and cooling stages 20, 22, 24 are configured to change the microstructure of the steel workpiece 12 and to produce a desired multiphase microstructure within the workpiece 12. In particular, the heat treatment and cooling stages 20, 22, 24 are configured to produce a multiphase microstructure within the workpiece 12 that includes a dispersed phase of retained austenite (γ-Fe) and a matrix phase of ferrite (α-Fe) after the workpiece 12 has been cooled to ambient temperature. In some situations, the heat treatment and cooling stages 20, 22, 24 also may result in the formation of one or more of the following additional phases within the steel workpiece 12 at ambient temperature: martensite, bainite, pearlite, and/or cementite. These additional phases may account for less than 10 vol. % of the steel workpiece 12, or more preferably less than 5 vol. % of the steel workpiece 12. The resulting multiphase microstructure of the steel workpiece 12 may provide the workpiece 12 with a combination of high strength and ductility, which may be attributed to a phenomenon commonly referred to as transformation-induced plasticity (TRIP). More specifically, during plastic deformation of the heat-treated multiphase steel workpiece 12, the austenite grains within the microstructure of the steel workpiece 12 may transform into relatively hard, brittle grains of martensite. This austenite-to-martensite transformation may help absorb energy and improve the work hardening capacity of the workpiece 12, which may help delay the onset of localized necking and allow for higher uniform elongation. At the same time, the relatively soft ferrite grains within the multiphase microstructure of the heat treated steel workpiece 12 may provide the workpiece 12 with excellent ductility.

The heat treatment and cooling stages 20, 22, 24 also are configured to tailor the microstructure of the steel workpiece 12 so that subsequent deformation of the heat-treated multiphase steel workpiece 12 can proceed in a substantially homogenous or uniform manner, without the undesirable formation of Lüders bands, which may appear as roughness, wrinkles, or depressions on the surface of a shaped part. Without intending to be bound by theory, it is believed that the formation of Lüders bands within the heat-treated multiphase steel workpiece 12 can be avoided—without impairing the high strength and ductility of the workpiece 12—by encouraging the austenite-to-martensite transformation to occur prior to (i.e., at lower stress than) the plastic deformation of the grains of ferrite within the microstructure of the steel workpiece 12. And it has been discovered that the favorability of the austenite-to-martensite transformation may be increased by increasing the grain size of the austenite phase (thereby lowering the stability of the austenite phase) while maintaining the ultrafine grain size of the ferrite phase (thereby maintaining the strength of the ferrite phase and the mechanical strength of the workpiece 12).

The temperatures at which the heat treatment and cooling stages 20, 22, 24 of the process 10 are performed are described herein with respect to the lower austenite transformation temperature A₁ (i.e., the initial temperature at which austenite grains begin to form within the microstructure of the workpiece 12 upon heating) and upper austenite transformation temperature A₃ (i.e., the temperature at which the transformation of ferrite grains to austenite grains within the microstructure of the workpiece 12 is complete upon heating). These transformation temperatures A₁, A₃ may vary depending upon the specific chemical composition of the steel workpiece 12. Such temperatures A₁, A₃ can be readily determined by persons or ordinary skill in the art based upon the chemical composition of the steel workpiece 12.

The first heat treatment stage 20 may comprise a first heating step and a subsequent first soaking step. During the first heating step, the steel workpiece 12 is heated at a suitable rate to a first temperature (T₁) greater than or equal to its lower austenite transformation temperature A₁ and less than its upper austenite transformation temperature A₃ (i.e., A₁≤T₁≤A₃). For example, the steel workpiece 12 may be heated to a first temperature greater than or equal to its A₁ temperature, but less than or equal to 50° C. below its A₃ temperature (i.e., A₁≤T₁≤A₃−50° C.). In one specific example, the steel workpiece 12 may be heated to a first temperature within a range of about 50° C. above its A₁ temperature to about 150° C. above its A₁ temperature (i.e., A₁+50° C.≤T₁≤A₁+150° C.). The rate of heating may be based on practical heating practices and may depend on the composition of the steel workpiece 12, its mass, and/or its thickness. Heat may be applied to the steel workpiece 12 during the first heat treatment stage 20 via convection, conduction, radiation, induction, or a combination thereof.

In the subsequent first soaking step, the temperature of the workpiece 12 is maintained at the first temperature T₁ for a time sufficient to produce a microstructure within the workpiece 12 that would comprise a plurality of austenite grains within a ferrite matrix, if the workpiece 12 were cooled down to ambient temperature after the first heat treatment stage 20 was complete. For example, the workpiece 12 may be maintained at the first temperature T₁ for a time sufficient to produce a microstructure within the workpiece 12 that would comprise 5-40 vol. % austenite and 60-95 vol. % ferrite at ambient temperature. In one specific example, the workpiece 12 may be maintained at the first temperature T₁ for a time sufficient to produce a microstructure within the workpiece 12 that would comprise 10-25 vol. % austenite and 75-90 vol. % ferrite at ambient temperature. If, prior to the first heat treatment stage 20, the steel workpiece 12 exhibited a martensitic microstructure, the temperature of the steel workpiece 12 may be maintained at the first temperature T₁ for a time sufficient to transform the grains of martensite in the microstructure of the steel workpiece 12 into grains of ferrite and austenite.

In addition, the workpiece 12 may be maintained at the first temperature T₁ during the first soaking step for a time sufficient to produce grains of austenite within the microstructure of the steel workpiece 12 that would have an average grain diameter in the range of 0.2 μm to 1.8 μm, if the workpiece 12 were cooled down to ambient temperature after the first heat treatment stage 20 was complete. In one form, the workpiece 12 may be maintained at the first temperature T₁ for a time sufficient to produce grains of austenite within the microstructure of the steel workpiece 12 that would have an average grain diameter in the range of 0.2 μm to 0.9 μm at ambient temperature. The workpiece 12 also may be maintained at the first temperature T₁ for a time sufficient to produce grains of ferrite within the microstructure of the steel workpiece 12 that would have an average grain diameter in the range of 0.2 μm to 1.8 μm at ambient temperature. In one form, the workpiece 12 may be maintained at the first temperature T₁ for a time sufficient to produce grains of ferrite within the microstructure of the steel workpiece 12 that would have an average grain diameter in the range of 0.2 μm to 0.9 μm at ambient temperature. The precise duration of the first heat treatment stage 20 may depend on the composition of the steel workpiece 12, its mass, and/or its thickness. For example, the overall duration of the first heat treatment stage 20, including the first heating step and the first soaking step, may be in the range of one (1) second to one-hundred (100) hours. In one form, the overall duration of the first heat treatment stage 20, including the first heating step and the first soaking step, may be on the order of a few minutes. For example, the overall duration of the first heat treatment stage 20 may be in the range of 2-4 minutes (120-240 seconds), 2.5-3.5 minutes (150-210 seconds), or about 3 minutes (180 seconds).

The second heat treatment stage 22 is performed after the first heat treatment stage 20 and may comprise a second heating step and a subsequent second soaking step. During the second heating step, the steel workpiece 12 is heated at a suitable rate to a second temperature T₂ greater than the first temperature T₁ (i.e., T₂>T₁). The second temperature T₂ may comprise a temperature within a range of greater than or equal to 100° C. below its A₃ temperature to less than or equal to 20° C. above its A₃ temperature (i.e., A₃−100° C.≤T₂≤A₃+20° C.). For example, the steel workpiece 12 may be heated during the second heating step to a temperature greater than or equal to 20° C. below its A₃ temperature to a temperature less than or equal to its A₃ temperature (i.e., A₃−20° C.≤T₂≤A₃). The rate of heating may be based on practical heating practices and may depend on the composition of the steel workpiece 12, its mass, and/or its thickness. Heat may be applied to the steel workpiece 12 during the second heat treatment stage 22 via convection, conduction, radiation, induction, or a combination thereof.

In the subsequent second soaking step, the temperature of the workpiece 12 is maintained at the second temperature T₂ for a time sufficient to increase the average grain diameter of the austenite grains within the microstructure within the steel workpiece 12, without significantly increasing the average grain diameter of the ferrite grains. As such, the second heat treatment stage 22 will have the effect of increasing the volume fraction of austenite within the microstructure of the steel workpiece 12 and possibly reducing the volume fraction of ferrite within the microstructure of the steel workpiece 12. The duration of the second heat treatment stage 22 may depend on the composition of the steel workpiece 12, its mass, and/or its thickness. In one form, the overall duration of the second heat treatment stage 22, including the second heating step and the second soaking step, may be on the order of a few minutes. For example, the overall duration of the second heat treatment stage 22 may be in the range of one (1) second to 1000 seconds (or about 16.5 minutes). In one specific example, the overall duration of the second heat treatment stage 22 may be in the range of 2-4 minutes (120-240 seconds), 2.75-3.75 minutes (165-225 seconds), or about 3.25 minutes (195 seconds).

The cooling stage 24 is performed after the second heat treatment stage 22 is complete. In the cooling stage 24, the steel workpiece 12 is cooled down to ambient temperature at a rate sufficient to avoid transformation of a major portion of the austenite grains to martensite (or other austenite decomposition products), and thereby retain a major portion of the austenite grains formed during the first and second heat treatment stages within the microstructure of the steel workpiece 12. The cooling rate may be based on practical cooling practices. The steel workpiece 12 may be cooled down to ambient temperature by air cooling, water cooling, or high pressure liquid nitrogen. After the steel workpiece 12 has been brought down to ambient temperature during the cooling stage 24, the workpiece will comprise a dispersed phase of retained austenite and a matrix phase of ferrite. As a result of the heat treatment and cooling stages 20, 22, 24, the steel workpiece 12 will exhibit a microstructure that comprises greater than 30 vol. % retained austenite at ambient temperature. In one form, the steel workpiece 12 may exhibit a microstructure that comprises ≥30 vol. % and ≤40 vol. % retained austenite. The ferrite matrix phase may comprise 40 vol. % to 70 vol. % of the microstructure of the steel workpiece 12 at ambient temperature.

The grains of austenite retained within the microstructure of the steel workpiece 12 after the cooling stage 24 may have an average grain diameter in the range of 0.3 μm to 2.5 μm at ambient temperature. For example, the grains of austenite retained within the microstructure of the steel workpiece 12 after the cooling stage 24 may have an average grain diameter in the range of 0.4 μm to 1.0 μm at ambient temperature. The average grain diameter of the grains of ferrite may remain substantially unchanged and may be in the range of 0.2 μm to 1.8 μm at ambient temperature. In one form, the average grain diameter of the grains of ferrite within the steel workpiece 12 after the cooling stage 24 may be somewhat smaller than the average grain diameter of the grains of ferrite within the steel workpiece 12 prior to the second heat treatment stage 22. After the cooling stage 24, the diameter of some of the grains of austenite within the microstructure of the steel workpiece 12 may be larger than the average grain diameter of the grains of ferrite. For example, approximately 10-100% of the grains of austenite within the microstructure of the steel workpiece 12 may have a diameter that is larger than the average grain diameter of the grains of ferrite within the microstructure of the steel workpiece 12. The martensite start M_(S) temperature of the steel workpiece 12 after the cooling stage 24 may be lower than the M_(S) temperature of the steel workpiece 12 prior to the first heat treatment stage 20.

Referring now to FIG. 2, after the heat treatment and cooling stages 20, 22, 24 are complete and the steel workpiece 12 is recoiled on the recoiler 18, the steel workpiece 12 may be transported to a stamping or hot forming operation 50 and formed into a shaped steel part 62. In a first stage of the operation 50, the steel workpiece 12 may be uncoiled from an uncoiler 52 and cut by a pair of sheers 54 into a steel blank 56. Thereafter, the blank 56 may be positioned between a pair of upper and lower tool dies 58, 60 having opposed complementary surfaces. The upper tool die 58 is then lowered onto the lower tool die 60 to deform the blank 56 between the complementary surfaces of the upper and lower tool dies 58, 60. Thereafter, the upper tool die 58 is lifted away from the lower tool die 60 and the shaped steel part 62 is removed therefrom. As a result of the previously performed heat treatment and cooling stages 20, 22, 24, the shaped steel part 62 will not exhibit undesirable surface roughness or markings known as Lüders bands.

Referring now to FIG. 3, which depicts a graph of processing temperature versus time for a process for manufacturing a low carbon, medium manganese steel workpiece having a multiphase microstructure including a dispersed phase of retained austenite and a matrix phase of ferrite, in accordance with one embodiment of the present disclosure. For reference, dashed lines are drawn from the vertical temperature axis illustrating the M_(S) (100), A₁ (102), and A₃ (104) temperatures relative to the processing temperatures. The manufacturing process includes a hot rolling stage 110, a cold rolling stage 120, and a multistage heat treatment process. Like the process 10 described above with respect to FIG. 1, the multistage heat treatment process depicted in FIG. 3 also includes a first heat treatment stage 130, a second heat treatment stage 140, and a cooling stage 150. The details of stages 20, 22, 24 described above with respect to FIG. 1 apply equally to stages 130, 140, and 150 depicted here in FIG. 3 and thus will not be repeated.

As depicted in FIG. 3, in one embodiment, the second heat treatment stage 140 may be performed immediately after the first heat treatment stage 130. In other words, the steel workpiece 12 may be heated to a first temperature above its lower austenite transformation temperature A₁, but below its upper austenite transformation temperature A₃, maintained at the first temperature for a time sufficient to produce a microstructure within the workpiece 12 that includes a plurality of austenite grains within a ferrite matrix, and then immediately heated to a second temperature above the first temperature. In such case, the steel workpiece 12 may not be cooled or subjected to any intermediate treatment stages between the first and second heat treatment stages 130, 140.

Referring now to FIG. 4, which depicts a graph of processing temperature versus time for a process for manufacturing a low carbon, medium manganese steel workpiece having a multiphase microstructure including a dispersed phase of retained austenite and a matrix phase of ferrite, in accordance with another embodiment of the present disclosure. For reference, dashed lines are drawn from the vertical temperature axis illustrating the M_(S) (200), A₁ (202), and A₃ (204) temperatures relative to the processing temperatures. The manufacturing process includes a hot rolling stage 210, a cold rolling stage 220, and a multistage heat treatment process. Like the process 10 described above with respect to FIG. 1, the multistage heat treatment process depicted in FIG. 4 also includes a first heat treatment stage 230, a second heat treatment stage 240, and a cooling stage 250. The details of stages 20, 22, 24 described above with respect to FIG. 1 apply equally to stages 230, 240, and 250 depicted here in FIG. 4 and thus will not be repeated.

The multistage heat treatment process depicted in FIG. 4 also includes an intermediate cooling stage 260 between the first and second heat treatment stages 230, 240. As depicted in FIG. 4, in one form, the steel workpiece may be heated and maintained at a first temperature above its lower austenite transformation temperature A₁, but below its upper austenite transformation temperature A₃ during the first heat treatment stage 230, and then cooled down to ambient temperature during the intermediate cooling stage 260. The steel workpiece may be maintained at ambient temperature during the intermediate cooling stage 260 for any suitable amount of time. Thereafter, the steel workpiece 12 may be reheated to a second temperature above the first temperature to initiate the second heat treatment stage 240. Instead of cooling the steel workpiece 12 down to ambient temperature during the intermediate cooling stage 260, alternatively the steel workpiece 12 may be cooled down to any other desired temperature less than that of the first temperature and maintained at such temperature for any suitable amount of time prior to initiating the second heat treatment stage 240.

EXAMPLES

The microstructure and deformation behavior of two hot rolled, cold rolled, and heat treated steel sheets comprising, by weight, 0.14% carbon (C), 7.0% manganese (Mn), and 0.2% silicon (Si) were evaluated. The steel sheets initially had a martensite start M_(S) temperature of about 230° C., a lower austenite transformation temperature A₁ of about 500° C., and an upper austenite transformation temperature A₃ of about 710° C. Tensile tests were performed on samples cut from the steel sheets along a rolling direction and along a transverse direction crossing the rolling direction.

Example 1

A first one of the steel sheets was subjected to a single stage heat treatment process. Starting at ambient temperature, the steel sheet was heated at a suitable heating rate to a temperature T of about 620° C. (T=A₃-90° C.), and maintained at such temperature for 3 minutes (180 seconds). Thereafter, the steel sheet was air cooled down to ambient temperature.

FIG. 5 is an electron backscattered diffraction (EBSD) map depicting the microstructure of the heat treated steel sheet. As shown, the heat treated steel sheet exhibits a multiphase microstructure including a plurality of dispersed austenite grains (white) and a matrix phase of ultrafine grains of ferrite (black). Austenite grains having high angle grain boundaries (>15°) are shown as black lines and ferrite grains having high angle grain boundaries (>15°) are shown as white lines. The EBSD map indicates that the heat treated steel sheet comprises about 20 vol. % retained austenite and about 80 vol. % ferrite. The austenite grains in the steel sheet have an average grain diameter of about 0.33 μm and the ferrite grains have an average grain diameter of about 0.6 μm.

Uniaxial tensile tests were performed on samples of the heat treated steel sheet. FIG. 7 depicts a graph of Engineering Stress (MPa) vs. Engineering Strain (%) for a sample cut from the steel sheet in the rolling direction (300). As shown, the stress-strain curve of the steel sample initially follows a generally straight path, which represents the region of elastic or reversible deformation of the steel sample. Deformation along this straight path continues until the steel sample reaches an upper yield point (302) of about 1110 MPa, at which point the steel sample abruptly yields and plastic deformation begins. The transition from elastic to plastic deformation of this steel sample is notably discontinuous. After reaching the upper yield point 302, the stress-strain curve drops to a lower yield point (304) of about 1050 MPa, at which point deformation of the sample continues at generally constant stress. The strain or elongation experienced by the steel sample within this plateau of generally constant stress or load is known as Lüders strain or yield point elongation and is indicative of heterogeneous plastic deformation within the sample due to the formation and propagation of Lüders bands or stretcher strains. In this Example, the steel sample exhibits a Lüders strain of greater than 20%. The steel sample abruptly yields again at fracture (306). The total elongation of this steel sample at fracture 306 was about 38%.

Although not depicted in FIG. 7, uniaxial tensile tests performed on a sample of the steel sheet cut along the transverse direction resulted in an elongation at fracture of about 10%.

Example 2

A second one of the steel sheets was subjected to a dual stage heat treatment process. Starting at ambient temperature, in the first heat treatment stage, the steel sheet was heated at a suitable heating rate to a first temperature T₁ of about 620° C. (T₁=A₃−90° C.) and maintained at such temperature for 3 minutes (180 seconds). Immediately thereafter, in the second heat treatment stage, the steel sheet was heated to a second temperature T₂ of about 710° C. (T₂=A₃) and maintained at such temperature for 3.25 minutes (195 seconds). The steel sheet was not cooled between the first and second heat treatment stages. After completion of the second heat treatment stage, the steel sheet was air cooled down to ambient temperature.

FIG. 6 is an electron backscattered diffraction (EBSD) map depicting the microstructure of the heat treated steel sheet. As shown, the heat treated steel sheet exhibits a multiphase microstructure including a plurality of dispersed austenite grains (white) and a matrix phase of ultrafine grains of ferrite (black). Austenite grains having high angle grain boundaries (>15°) are shown as black lines and ferrite grains having high angle grain boundaries (>15°) are shown as white lines. The EBSD map indicates that the heat treated steel sheet comprises about 35 vol. % retained austenite and about 65 vol. % ferrite. The austenite grains in the steel sheet have an average grain diameter of about 0.49 μm and the ferrite grains have an average grain diameter of about 0.51 μm.

Uniaxial tensile tests were performed on samples of the heat treated steel sheet. FIG. 7 depicts a graph of Engineering Stress (MPa) vs. Engineering Strain (%) for a sample cut from the steel sheet in the rolling direction (400). As shown, plastic yielding of the steel sample initially begins at about 800 MPa and is followed by continuous yielding thereafter, with the steel sample experiencing a gradual increase in yield stress with increasing strain up to an ultimate tensile stress of about 1300 MPa. The transition from elastic to plastic deformation of this steel sample is generally continuous. In particular, the stress-strain curve of this steel sample does not exhibit an upper yield point followed by a lower yield point or a period of prolonged deformation (increasing strain) at generally constant stress (Lüders strain or yield point elongation). This indicates that deformation of this steel sample proceeded substantially homogenously throughout the sample during the tensile test, without the formation or propagation of Lüders bands or stretcher strains. In addition, the continuous curve and slope of the stress-strain curve indicates that the steel sample experienced work or strain hardening as a result of the applied load during the tensile test.

A sample of this steel sheet cut along the rolling direction exhibited an elongation at fracture of about 27%, and a sample of this steel sheet cut along the transverse direction exhibited an elongation at fracture of about 21%. 

What is claimed is:
 1. A method of heat treating a steel workpiece having a polycrystalline microstructure, the method comprising the following steps: (a) heating the steel workpiece to a first temperature greater than or equal to its lower austenite transformation temperature A₁, but less than its upper austenite transformation temperature A₃ to transform the microstructure of the steel workpiece into a multiphase microstructure including grains of ferrite and grains of austenite having an average grain diameter; (b) heating the steel workpiece to a second temperature greater than the first temperature to increase the average grain diameter of the grains of austenite in the steel workpiece; and (c) cooling the steel workpiece to ambient temperature at a rate sufficient to retain a major portion of the grains of austenite obtained during steps (a) and (b) so that the microstructure of the steel workpiece comprises a retained austenite phase dispersed within a ferrite matrix phase at ambient temperature.
 2. The method set forth in claim 1 wherein the steel workpiece comprises, by weight, 5-12% manganese (Mn) and 0.1-0.3% carbon (C).
 3. The method set forth in claim 1 wherein the steel workpiece is heated in step (a) to a first temperature less than or equal to 50° C. below the A₃ temperature of the steel workpiece.
 4. The method set forth in claim 1 wherein the steel workpiece is heated in step (b) to a second temperature greater than or equal to 100° C. below the A₃ temperature of the steel workpiece and less than or equal to 20° C. above the A₃ temperature of the steel workpiece.
 5. The method set forth in claim 1 wherein the steel workpiece is heated in step (a) for a duration in the range of one second to one-hundred hours, and wherein the steel workpiece is heated in step (b) for a duration in the range of one second to 1000 seconds.
 6. The method set forth in claim 1 further comprising: between steps (a) and (b), cooling the steel workpiece to a third temperature less than the first temperature.
 7. The method set forth in claim 1 wherein, after step (c), 10-100% of the grains of austenite in the steel workpiece have a diameter greater than an average grain diameter of the ferrite matrix phase.
 8. The method set forth in claim 1 wherein, after step (c), the retained austenite phase comprises at least 30 vol % of the microstructure of the steel workpiece.
 9. The method set forth in claim 1 wherein, after step (c), the ferrite matrix phase comprises at least 40 vol % of the microstructure of the steel workpiece.
 10. The method set forth in claim 1 wherein, after step (c), the steel workpiece comprises ≤10 vol % martensite, bainite, pearlite, and/or cementite.
 11. The method set forth in claim 1 wherein, after step (c), the steel workpiece is formed into a shaped part without exhibiting Lüders strain or yield point elongation during deformation thereof.
 12. The method set forth in claim 1 wherein the steel workpiece is in the form of a hot-rolled and cold-rolled steel sheet.
 13. A method of manufacturing a steel part comprising: providing a hot-rolled and cold-rolled steel sheet having a polycrystalline microstructure and comprising, by weight, 0.1-0.3% carbon (C) and 5-12% manganese (Mn); heating the steel sheet to a first temperature greater than or equal to its lower austenite transformation temperature A₁, but less than its upper austenite transformation temperature A₃ to transform the microstructure of the steel sheet into a multiphase microstructure including grains of ferrite and grains of austenite having an average grain diameter; heating the steel sheet to a second temperature greater than the first temperature to increase the average grain diameter of the grains of austenite in the steel sheet; cooling the steel sheet to ambient temperature at a rate sufficient to retain a major portion of the grains of austenite within the microstructure of the steel sheet so that the steel sheet comprises a retained austenite phase dispersed within a ferrite matrix phase at ambient temperature; and forming the steel sheet into a shaped steel part.
 14. The method set forth in claim 13 wherein the hot-rolled and cold-rolled steel sheet has a thickness in the range of 0.5 mm to 6 mm.
 15. The method set forth in claim 13 wherein the steel sheet is formed into the shaped steel part without exhibiting Lüders strain or yield point elongation during deformation thereof.
 16. The method set forth in claim 13 wherein an exterior surface of the shaped steel part does not exhibit Lüders bands or stretcher strain marks.
 17. The method set forth in claim 13 wherein the shaped steel part comprises an automotive vehicle body panel or support structure. 